1. Field of the Invention
The present invention relates to fiber optic couplers and particularly to a novel infrared fiber optic coupler, useful in multiplexing and demultiplexing optical data, creating components for communications systems including add/drop devices, amplifiers and oscillators, making hybrid or multi-element sensors, and for fiber interferometry. Optical couplers represent one of the fundamental building blocks for sophisticated optical fiber devices, which have not yet been made for mid-infrared (about 1.5-12 .mu.m) operation.
2. Description of the Related Art
Many processes exist and are employed to make fiber coupler devices which are suited for operation in the visible spectrum or the near-infrared (&lt;1.6 .mu.m), most commonly utilizing silica fibers; the fusion technique has been employed in making fiber couplers from silica glass fibers for over two decades. Yet these kind of devices are not available in the mid-infrared at present, largely due to the lack of methodology and knowledge for making basic components such as couplers from chalcogenide or other infrared-transmitting glass fibers, and due to the inadequate quality of the fiber that has been produced in the past.
With the availability of high-quality chalcogenide fiber, the fabrication of these devices has become feasible. Just as chalcogenide fibers have advantages in the infrared over standard silica glass fibers, devices made from these infrared fibers should show marked performance improvement over any device made with silica fiber. Infrared fiber devices, using fiber couplers, can now be fabricated for applications in chemical sensing, data transmission, and infrared spectroscopy.
There are three common elements to the majority of coupler fabrication processes which utilize thermal fusion: etching or surface preparation, heating, and mechanical bonding. There are several important differences in fusion coupling (defined as heating and mechanical bonding) of chalcogenide glasses versus silica glass, which can be summarized as follows:
1. Etching and Surface Preparation PA1 2. Thermal Mechanics PA1 3. Mechanical Considerations
a) Chalcogenide compounds cannot be etched with hydrofluoric acid, as is the common practice with silicates. Alternative etchants such as KOH must be used. PA2 b) In silicates, the surface oxidation is not a major concern during the fusion process, since most of these compounds contain oxygen as a primary constituent. In chalcogenides, surface oxides form an entirely new compound which inhibits wetting, surface contact, and which form an optical barrier in the waveguide. This layer can be removed in a reactive atmosphere or by plasma treatment. PA2 a) The melting and softening temperatures of the chalcogenide glass are substantially lower than the melting temperature of silica, so open flame heating systems cannot be used. PA2 b) Chalcogenides are much more susceptible to oxidation than silica, and thus the process must be performed in an inert atmosphere, other than oxygen-containing atmospheres (e.g. halogen atmospheres). PA2 c) The viscosity profile as a function of temperature is steeper in chalcogenides than in silica, so the range of temperatures over which the fusion can be achieved is narrower. The temperature or viscosity range for the fusion process is particularly important since these chalcogenide compositions are highly susceptible to interdiffusion of the core and cladding material at elevated temperatures. PA2 d) The higher vapor pressure of chalcogenides at elevated temperatures leads to greater volatilization of the glass constituents, the loss of which impairs the optical and mechanical performance of the fiber. This means that close control of temperature in this process is important and necessary. PA2 a) The more fragile chalcogenide fiber requires different handling and tensioning procedures than mandated by the established silica process, particularly when twisted to achieve overlap between the fibers. PA2 b) The compositional range of the chalcogenide glasses allows for a greater variation in the numerical aperture of the fiber, which determines the magnitude of the evanescent field outside the core. Since the coupling in these devices is largely evanescent, and driven by bends or twists in the fiber where light can escape more easily, this variability means that the chalcogenide fiber coupler fabrication process can tolerate a wider range of bend radii in the fiber and hence more or fewer twists of the fiber. PA2 c) The higher refractive index of chalcogenide glass ensures greater light guiding even in core-only material, so that slight air gaps in the coupler body result in much lower losses.
A multimode "coupler" made from chalcogenide glass has been fabricated previously, but this device employed core-only fiber, and did not actually fuse the fibers together. The performance of this device is also affected by atmospheric conditions. The device described in this application uses core-clad fiber, which has inherently lower losses and is greatly preferred over core-only fiber in making sensing and communications devices.